An optical film is used as a retardation film or a protective film for polarizing plates. The retardation film is used in liquid crystal displays and the like and has such functions as color correction, the expansion of the view angle and antireflection. A λ/4 plate and a λ/2 plate are known as the retardation film, and a polycarbonate, polyether sulfone and polysulfone are used as the materials of these plates. The λ/4 plate and the λ/2 plate made from these materials are characterized in that their retardation becomes larger as the wavelength becomes shorter. Therefore, the wavelength at which the λ/4 plate and the λ/2 plate can function is limited to a specific wavelength.
As means of controlling the wavelength at a broad band, there is known a method in which two or more birefringence films having different wavelength dependences of retardation are laminated together at a specific angle (JP-A 2-120804: Patent Document 1). This method needs the step of laminating together a plurality of retardation films and the step of adjusting the lamination angle, thereby causing a problem with productivity. Further, as the thickness of the whole retardation film becomes large, its light transmittance lowers with the result that it becomes dark therewith.
There has recently been proposed a method of controlling the wavelength at a broad band with a single film without laminating together a plurality of films as described above (Japanese Patent No. 3325560; Patent Document 2). In this method, a polycarbonate copolymer comprising a unit having positive refractive index anisotropy and a unit having negative refractive index anisotropy is used. However, this polycarbonate copolymer has a high melting temperature because it contains a unit derived from a fluorene-based bisphenol and readily produces a gel through its decomposition when it is molten. Further, it has a high glass transition temperature (Tg), a high temperature is required to stretch a film thereof, and special processing equipment different from that of the prior art is needed. It has a high photoelastic constant and large stress birefringence and involves such a problem as light slipping when it is used as a retardation film.
Meanwhile, a polycarbonate copolymer which is used in an optical fiber or an optical disk, comprises an aliphatic diol and has a low photoelastic constant has already been proposed (Japanese Patent No. 3160209: Patent Document 3). However, the stretching and chromatic dispersibility of a film are not investigated in this document. The photoelastic constant of the polycarbonate copolymer described in this document must be further reduced when it is used as a retardation film or a protective film for polarizing plates.
There has also been reported a retardation film which is made of a polycarbonate copolymer containing a fluorene ring and an isosorbide component and has a low photoelastic constant (WO06/041190: Patent Document 4). Since this polycarbonate copolymer is a terpolymer, it is necessary to control the ratio of three components precisely in order to control its chromatic dispersibility and therefore, it is not easy to produce the polycarbonate copolymer stably. Since it has low heat stability, its molecular weight is apt to lower at the time of melting.
There has further been proposed a retardation film made of a polycarbonate copolymer containing a fluorene-based bisphenol skeleton (WO01/009649: Patent Document 5, JP-A 2006-323254: Patent Document 6).
There has further been proposed a polarizing plate protective film made of a polycarbonate copolymer containing a fluorene-based bisphenol skeleton (Japanese Patent No. 3995387: Patent Document 7).
(Patent Document 1) JP-A 2-120804
(Patent Document 2) Japanese Patent No. 3325560
(Patent Document 3) Japanese Patent No. 3160209
(Patent Document 4) WO06/041190
(Patent Document 5) WO01/009649
(Patent Document 6) JP-A 2006-323254
(Patent Document 7) Japanese Patent No. 3995387